The origin of the most expensive diamonds on Earth is still a mystery

In December 2016, scientists discover that the largest and most valuable gems are formed 700 kilometers underground, the finding sheded light on the functioning of the earth’s mantle and what they tell us about the Earth.

Record Unit 371 Box 4 Folder January 1983 © Wikipedia CC 00

“They are a girl’s best friends,” says the famous song about diamonds. Especially if they have a giant size and exceptional quality, like the famous “The Constellation” (813 carats), the most expensive in the world; the ‘Cullinan’ (3,100 carats), carved into various British Crown Jewels; or the “Koh-i-Noor”, found in India, which also decorates the crown of the late Queen Mother.

Researchers from the Gemological Institute of America (GIA) studied similar stones and, as explained in the journal “Science”, they had discovered that their extraordinary rarity lies not only in their characteristics and proportions, but also in their origins: They have come from the depths of the Earth.

Some of the world’s largest and most valuable diamonds, such as the ‘Cullinan’ or the ‘Lesotho Promise’, exhibit a distinct set of physical characteristics that have led many to view them as separate from more common ones.

However, exactly how these diamonds are formed and what they tell us about Earth has been a mystery until now. New research shows that jewels such as ‘Cullinan’ sometimes have tiny metallic inclusions trapped inside, accompanied by traces of methane and fluid hydrogen.

In addition to the metallic ones, some of these rare diamonds contain mineral inclusions that show that they formed at extreme depths, probably between 360 and 750 km underground, in the convection mantle. This depth is much greater than that of the most diamonds, which form at the bottom of continental tectonic plates, about 150 to 200 km underground.

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